Combining Invasive Cardiopulmonary Exercise Testing with Computational Fluid Dynamics to Better Understand LVAD Fluid Mechanics during Exercise

Abstract

<h3>Purpose</h3> Invasive cardiopulmonary exercise testing (iCPET) is helpful in evaluating dyspnea but is infrequently utilized in patient supported by left ventricular assist devices (LVAD). Computational fluid dynamics (CFD) allows for a more thorough analysis of fluid mechanics including shear stress (SS) and shear rate (SR). The additive information from iCPET and CFD may allow for optimal understanding of the pump-patient interaction during exercise. <h3>Methods</h3> Hemodynamic and ventilatory gas measurements were recorded as part of an iCPET study on a patient with a HeartMate 3 (HM3). Exercise using a bicycle ergometer by increments of 20 watts every 2 minutes was performed. An anatomical model of the vasculature was reconstructed from computed tomography (CT) images, and the LVAD outflow cannula was used as the inflow boundary. The LVAD outflow was calculated separately using a lumped-parameter-model (LPM) of the circulation. Ventricular volumes were reconstructed from CT and the patient's hemodynamic tracings. The contribution of flow from the HM3 was implemented in the LPM via published H-Q curves. CFD simulations were then carried out over the stages of exercise. Blood velocities, SS and SR were quantified in the vascular beds. <h3>Results</h3> Compared to rest, blood volume exposed to SR >1000 s⁻¹ increased 2.8 fold (0.47 to 1.31 cm³), while average SS increased 1.45 fold (2.61 to 3.79 Pa) in the cannula, 1.48 fold (0.89 to 1.31 Pa) in the ascending aorta, and 1.61 fold (0.67 to 1.08 Pa) in the aortic arch and cerebral circulation. Compared to rest, peak velocities increased nearly 5 fold in the aortic arch. <h3>Conclusion</h3> CFD applied to iCPET provides valuable information about variations in flow to the vasculature with exercise as well as shear rate and shear stress augmentation. This patient-specific information may help with optimization of LVAD speeds to meet the physiological needs of exercise.